1. Field of the Invention
The present invention relates to an image forming apparatus and a method of controlling the same. More particularly, the present invention relates to an electrophotographic image forming apparatus such as a printer and a color copying machine, and a method of controlling the same.
2. Description of the Related Art
Tandem color image forming apparatuses are recently growing in number, each of which includes developing units and photosensitive drums equal in number to color materials and sequentially transfers images of different colors onto an image conveyance belt or a printing medium, thereby increasing the image forming speed in the electrophotographic color image forming apparatus. The tandem color image forming apparatus has a plurality of factors to cause registration errors, as is already known, and various measures have been proposed for the respective factors.
One of the factors is the nonuniformity or a position error of the lens in an optical scanning apparatus, or an assembly position error of the optical scanning apparatus to the color image forming apparatus. This inclines or curves the scanning lines. Since the degree of inclination or curve changes between the colors, a registration error occurs.
Japanese Patent Laid-Open No. 2002-116394 describes the following measure for such a registration error. First, in the process of assembling the optical scanning apparatus, the magnitude of the curve of the scanning lines is measured by using an optical sensor. The lens is mechanically rotated to adjust the curve of the scanning lines, and then fixed by an adhesive.
Japanese Patent Laid-Open No. 2003-241131 describes the following method. First, in the process of assembling the optical scanning apparatus to the color image forming apparatus main body, the magnitude of the inclination of the scanning lines is measured by using an optical sensor. The optical scanning apparatus is mechanically inclined to adjust the inclination of the scanning lines, and then assembled to the color image forming apparatus main body.
Japanese Patent Laid-Open No. 2004-170755 describes the following method. First, the magnitudes of the inclination and curve of the scanning lines are measured by using an optical sensor. Bitmap image data is corrected to cancel the inclination and curve, and a corrected image is formed. This method electrically corrects the inclination and curve by processing image data without using any mechanical adjusting members or adjustment process in assembly. It is therefore possible to cope with the registration error at a lower cost than the methods described in Japanese Patent Laid-Open Nos. 2002-116394 and 2003-241131.
This electrical registration error correction is divided into pixel-by-pixel correction and correction in an amount smaller than a pixel. In pixel-by-pixel correction, each pixel is offset in the sub-scanning direction in accordance with the inclination and curve correction amounts. In correction in an amount smaller than a pixel, the tone value of bitmap image data is corrected in the preceding and subsequent pixels in the sub-scanning direction. The correction in an amount smaller than a pixel eliminates unnatural steps at the offset boundary caused by the pixel-by-pixel correction and smoothens the image.
However, registration error correction using the method of Japanese Patent Laid-Open No. 2004-170755 can cause an uneven density in a high density image.
The uneven density in a high density image will be described with reference to FIG. 13. An input image 501 is a thin line of 1 dot. An image 502 obtained by executing correction of out of color registration for the input image 501 is actually formed. Although the input image 501 is a thin line image with a uniform density, the output image after correction of out of color registration is a thin line image with an uneven density 503. This is because a general electrophotographic image forming apparatus is not good at forming isolated pixels while maintaining the proportional relationship between the image tone value and the actual image density value. In a high density image including such a thin line, this effect comes to the fore as an uneven density.
On the other hand, an image forming apparatus using an electrophotographic image forming process readily causes an image density fluctuation depending on conditions such as the use environment and the number of printed sheets. Especially, in a color image forming apparatus for executing color printing by superimposing toner images of a plurality of colors, when the image density of each color fluctuates, the color balance (so-called tint) fluctuates. It is therefore important to suppress the density fluctuation.
Many recent color image forming apparatuses obtain a stable image by controlling the image density. For example, a toner image for detection (to be referred to as a toner patch hereinafter) is formed as a test image on an image carrier such as a photosensitive body or an intermediate transfer member or on a transfer material carrier such as a transfer belt. An optical sensor detects the toner amount of the toner patch. The detection result is fed back to the exposure amount, developing bias, and the like, thereby controlling the image density.
The toner image for detection to be used for the image density control preferably uses a pattern that sensitively changes the density in response to the factors of density fluctuation. In many cases, a high density pattern including a larger number of pattern lines than a pattern for normal image formation for image formation on a printing medium fed from a feed tray is used.
Hence, when the above-described correction of out of color registration is performed, an uneven density may occur in the toner image pattern for detection used for image density control. Needless to say, if the toner image for detection used for image density control has an uneven density, the result of image density control becomes unstable, and consequently, the image density of a printed image fluctuates.
It is an object of the present invention to prevent fluctuations of the density of a formed image by stabilizing the result of image density control in an image forming apparatus for executing electrical registration error correction.